The herpesviruses are a widespread family of large viruses containing double-stranded DNA, causing disease in mammals, birds, reptiles, fish and amphibians.
The double-stranded DNA codes for about 30-35 polypeptides, depending on the genus. The molecular weight of the DNA greatly varies between 120 kbp in Channel Cat Fish Herpesvirus to 230 kbp in Human Cytomegalovirus. The GC-content of the DNA varies even more; from 32% in Dog Herpesvirus to 72-73% in Bovine Herpesvirus.
The viral agent s causing Infectious Bovine Rhinotracheitis (IBR), Infectious Pustular Vulvovaginitis (IPV) and Infectious Balanoposthitis (IBP) belong to the Herpesviruses, more specifically to the sub-family Alphaherpesvirinae (Roizman, B. In Fields, B. N., Knipe, D. M. (ed.) Fundamental Virology. Ravens Press, New York: 841-847 (1991)).
They are all designated Bovine Herpesvirus I (Roizman et al.; Intervirology 16: 210-217 (1981)), and more recently Bovid Herpesvirus I (Ludwig, H. in B. Roizman (Ed.) The Herpesviruses. Plenum Press New York, vol. 2: 135-214 (1983)). In cross-neutralisation tests BHV-I isolates all exhibit only one serotype, regardless their origin from IBR or IPV/IBP cases (Gillespie et al.; Cornell Vet. 49: 288-297 (1959), McKercher et al.; Can. J. Comp. Med. 23: 320-328 (1959)). Restriction enzyme analysis of large numbers of different isolates has revealed differences in restriction site patterns, but no correlation exists between the patterns and the origin of the isolates. Therefore it is now generally accepted that the site of infection with BHV-I determines the effects of the disease, whereas the causative agent is in all cases the Bovine Herpesvirus I.
Transmission of Infectious Bovine Rhinotracheitis can occur for instance a) by animals that are acutely infected with or without showing clinical signs, and that are still shedding virus, and b) by animals that are latently infected and start shedding recurrent virus after stress.
Spread of IPV/IBP occurs e.g. by contaminated semen, natural service, the teaser bull, or by the herdsmen.
The most common manifestation of BHV-I infection is bovine rhinotracheitis which varies from a mild respiratory disease to a severe infection of the entire respiratory tract.
From an economical point of view, IBR is also the most dramatic manifestation of BHV-I infection.
Another relatively important form of BHV-I infection is the genital form, leading to pustular vulvovaginitis in cows and, comparatively, to infectious balanoposthitis in bulls.
Although seen less frequently, the virus is also associated with abortion, conjunctivitis and encephalitis. BHV-I is widely spread among cattle in all continents, but its host range is limited. Many wild species have been found seropositive, but distinct clinical signs have only been observed in cattle.
Although IBR and IPV have been known in Europe for more than a century, it appears that wild ruminants in Africa and in zoos are the true reservoir of the virus.
Morbidity rates in both IBR and IPV are usually 100% provided that the animals are in close contact (for IPV).
If morbidity is &lt;100% then the animals are protected by antibodies. Fatalities are rare however, in the case of IBR and IPV. In young calves however, death as a result of BHV-I induced encephalitis is common. Next to this, several strains of BHV-I have shown to induce abortion. Due to secondary bacterial infections, pneumoniae and enteritis may occur (Bielefeld-Ohmann et al.; J. Infect. Diseases 151: 937-947 (1985))
Immunity against Herpesviruses is depending on at least two mechanisms: a) the induction of neutralising antibodies and complement-dependent lysis for the inactivation of free virus, and b) the induction of cytotoxic T-cells for the elimination of virus-infected cells.
At present, in general, cattle can be protected against infection by these pathogenic micro-organisms with live vaccines, inactivated vaccines and subunit vaccines.
However these types of vaccines may suffer from a number of drawbacks. The use of attenuated vaccines always carries the risk of inoculation of animals with inadequately attenuated pathogenic micro-organisms. In addition, the attenuated micro-organisms may revert to a virulent state resulting in disease of the inoculated animals and, consequently, spread of the pathogens to other animals. Moreover, a problem with combined live viral vaccines is the mutual influence of the antigenic components resulting in a decrease of potency of one or more of the components.
Inactivated vaccines are generally considered safe, but they generally induce only a low level of immunity, requiring repeated immunizations. Furthermore, the neutralisation inducing antigenic determinants of the pathogens may become altered by the inactivation treatment, decreasing the immunising potency of the vaccine. In addition, cellular immunity is to a lesser extend triggered by inactivated vaccines.
Subunit vaccines have the same drawbacks as inactivated vaccines, and additionally, their spectrum of protection is smaller since they contain less different antigenic determinants then the whole organism.
It is an object of the present invention to provide a safe live BHV-1 mutant which can be used not only for the preparation of a vaccine against Bovine Herpesvirus I, but also as a carrier of genetic information for antigenic determinants of other infectious diseases of cattle, which obviates any potential risk associated with the use of a live attenuated pathogen. Therefore the present invention is concerned with a vaccine, which stimulates both the cellular and humoral immune system in a potent way without the explicit need of an adjuvant and which offers the possibility of a multivalent vaccine without the risk of adverse mutual interference of different antigenic components.